The present invention relates to a well testing apparatus and a method of well testing, both being for determining properties of oil and gas wells.
Well testing methods are used in the oil and gas industry to measure the properties of oil wells and to monitor ongoing performance of the wells. It is desirable to determine properties of a well such as gas-oil ratio (GOR), water cut (WC), skin, permeability thickness, productivity index (PI), inflow performance relationship (IPR) and reservoir properties. Well tests are performed on a regular basis in order to monitor well performance and to meet regulatory requirements. Data obtained by the tests can be used to optimise production, for example maximisation of oil production within constraints on water and gas production. An oil reservoir will typically be accessed by multiple wells, each connected to a production header that combines the flow from each well and conveys the fluid stream from the well to a production separator where the oil, gas and water is separated. Since the flow from each well is combined, the fluid stream at the separator provides hardly any information about individual wells. In order to obtain data for the individual wells a dedicated test header is used. The header includes valves enabling the flow from individual wells to be controlled, for example to shift flow from the production header to the test header. To test a well the production flow rate at the well is controlled and pressure data is gathered, which will relate to that well. The conventional test procedures involve measurements for one well at a time.
Various types of well tests are known. The different well test types are characterised by different flow steps and are used at different points in the life of the well. A drawdown test is carried out after a well is put on production either initially or when valves are re-opened after an extended shut in period. Pressures are monitored as the flow rate is changed from zero flow to production flow rates. For wells that are on production a build-up test is used. In a build-up test a producing well is shut in by closing the valves and pressure data is measured. Soon after a well is shut in, the fluid in the wellbore usually reaches a state in which bottomhole pressure rises smoothly and is easily measured. Multi-rate tests are also used, in which the flow is stepped between different rates. Testing of producing wells can be used to monitor and optimise well performance. There is also generally a regulatory requirement for routine testing to be carried out.
The pressure data from each well is analysed to determine the well properties of interest. Since the test state of the well differs from its normal production state then reconciliation of the data is required. For example, it is necessary to allow for changes in the gas-liquid equilibrium that occur during production when the flows from multiple wells are combined in the production header.
These conventional well testing methods are time consuming and expensive. Sequential testing of multiple wells takes a large amount of time, and the loss in production when the wells are shut in can be considerable. A typical set of build-up tests covering ten wells might require a week long test campaign with an eight hour shut in period for each well.